The role of ultraviolet radiation in the tanning of the human epidermis, but also in skin ageing, and in particular that of erythemogenic UV-B rays of wavelengths between 280 and 320 nm, which are the main cause of protocarcinogenesis, is known. It has also been demonstrated that UV-A rays, of wavelengths between 320 and 400 nm, which cause tanning of the skin, result in photo-aging of the skin. However, solar radiation reaching the earth's surface also comprises 50% of a not insignificant radiation, namely infrared radiation, of wavelengths between approximately 760 nm and 1 mm.
The incidence of infrared radiation on the human epidermis is hence naturally closely linked to exposure to the sun, but also to various other artificial light sources such as, for example, infra-red lasers and infra red light therapy.